Range Mask is a new addition to the local adjustment tools (Adjustment Brush, Graduated Filter, and Radial Filter) in Lightroom Classic (and Camera Raw 10.x), which provides a way to fine-tune the area being adjusted by color or luminance values within the photo. This means that you’ll only see the Range Mask option after you’ve applied one of the three local adjustments, and then scrolled to the bottom of the local adjustment panel.